A comparative study of different doped metal cations on the reduction, adsorption and activity of CuO/Ce0.67M0.33O2 (M = Zr4+, Sn4+, Ti4+) catalysts for NO + CO reaction

Abstract

A series of ceria-based solid solutions (Ce0.67Zr0.33O2, Ce0.67Sn0.33O2, Ce0.67Ti0.33O2) were synthesized by inverse co-precipitation, and then used as supports to prepare CuO/Ce0.67M0.33O2 (M=Zr4+, Sn4+, Ti4+) catalysts through wetness impregnation method. The obtained samples were investigated in detail by means of XRD, LRS, N2-physisorption, H2-TPR, XRF, XPS and in situ FT-IR techniques. The catalytic reduction of NO by CO as a model reaction was chosen to evaluate the catalytic performance of these samples. These results suggest that: (1) the reduction of CuO/Ce0.67Zr0.33O2 is easier than CuO/Ce0.67Sn0.33O2 and CuO/Ce0.67Ti0.33O2 catalysts, which may be attributed to the difference in the electronegativity of dopant; (2) the reduced state Cu+ is present in CuO/Ce0.67Zr0.33O2 at ambient temperature due to the shifting of redox equilibrium (Cu2++Ce3+ ↔ Cu++Ce4+) to right; (3) the adsorbed NO species on CuO/Ce0.67Zr0.33O2 are more liable to desorb/transform/decompose than those on CuO/Ce0.67Sn0.33O2 and CuO/Ce0.67Ti0.33O2 samples. The results of catalytic performance show that Cu+/Cu0 species play a key role in NO reduction by CO, and the activity is mainly related to the electronegativity of dopant, the reduction and adsorption behaviors of these catalysts. Furthermore, a possible reaction mechanism (schematic diagram) is tentatively proposed to understand this reaction.